It is desired to accurately measure residual stress inside a structure. This is because the residual stress affects the strength and the life of the structure. A strain which causes the residual stress, such as a thermal strain or a plastic strain, is called inherent strain; an inherent strain method for calculating the residual stress from the inherent strain is proposed. The inherent strain method includes measuring a release strain (an elastic strain) caused by release of the residual stress, deriving a distribution of the inherent strain from the measured release strain according to an inverse analysis using a finite element method, and calculating a distribution of the residual stress according to a direct analysis using the finite element method.
For example, as a method of measuring residual stress in a shaft-like member on the basis of the inherent strain method, there is known a T-L method using a measurement piece (a T-piece) obtained by cutting a structure axially thereof and a measurement piece (an L piece) obtained by cutting the structure orthogonally to a cutting direction of the T-piece. Specifically, proposed is a method including measuring respective release strains concerning the T-piece and the L piece, deriving an inherent strain from the release strains using the finite element method in a model on a cylindrical coordinate, and further calculating residual stress (see, for example, Japanese Unexamined Patent Publication No. 2005-181172, “Measurement of Welding Residual Stress by Inherent Strain Method”).
The inherent strain method does not always require, because of the principle thereof, directly measuring a release strain of a portion desired to be measured. However, since measurement of a release strain involves an error, measurement of a release strain in a position with higher residual stress in an initial state allows prediction accuracy of residual stress to be improved. It is, therefore, desirable to set a larger number of measurement points in a portion containing a steep gradient of residual stress. However, because of a physical limit in narrowing an interval of cutting an object, the application of the conventional TL method may result in a situation where the portion containing a steep gradient of the residual stress is included in a single T-piece. This may cause calculation accuracy of residual stress to be insufficient depending on the shape of a structure.
The inconvenience is conspicuous especially in the case of measuring residual stress in a structure including a columnar shaft section and a tabular section (a flange) projecting outward radially beyond the outer circumferential surface of the shaft section, wherein a fillet surface for easing stress concentration is provided in a portion interconnecting the shaft section and the tabular section. Specifically, such a structure is likely to have a weakest part in the fillet surface, and therefore there is often applied a surface treatment technique for increasing the strength of the weakest part, that is, the fillet surface. The application of such a surface treatment technique to the fillet surface causes a concentrated distribution of residual stress near the fillet surface. According to the analysis using the cylindrical coordinate model based on the conventional T-L method, inclusion of the entire fillet surface in a single L-piece may cause an inconvenience of failing to analyze a local residual stress distribution near the fillet surface with sufficient accuracy.